Method for controlling the oxygen content in valve metal materials

ABSTRACT

A method for controlling oxygen content in valve metal materials. The method includes deoxidizing a valve metal material, typically tantalum, niobium, or alloys thereof, and leaching the material in an acid leach solution at a temperature lower than room temperature. In one embodiment of the present invention, the acid leach solution is prepared and cooled to a temperature lower than room temperature prior to leaching the deoxidized valve metal material. The method of the present invention has been found to lower both the oxygen and fluoride concentrations in valve metal materials, as the use of reduced acid leach temperatures provide lower oxygen for a given quantity of a leach acid, such as hydrofluoric acid.

This application is a continuation of application Ser. No. 08/628,878,filed Apr. 5, 1996, entitled METHOD FOR CONTROLLING THE OXYGEN CONTENTIN VALVE METAL MATERIALS, now U.S. Pat. No. 5,993,513, the disclosure ofwhich is incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of Invention

The present invention relates to a method of controlling the oxygencontent in valve metal materials and, more particularly, to a method ofcontrolling the oxygen content in powders of tantalum, niobium, andalloys thereof, useful in the production of capacitors, and in sinteredanode bodies made from tantalum, niobium, and alloys thereof.

2. Description of the Related Art

Valve metals may be used to form wrought products, such as bars, plates,sheets, wires, tubes and rods, and preforms for subsequentthermo-mechanical processing. In addition, capacitors can bemanufactured by compressing agglomerated tantalum powders to form apellet, sintering the pellet in a furnace to form a porous body(electrode), which is sometimes followed by deoxidation of the electrodeby reaction with a reactive metal, such as magnesium, and thensubjecting the body to anodization in a suitable electrolyte to form acontinuous dielectric oxide film on the sintered body.

As is known to those skilled in the art, valve metals generally includetantalum, niobium, and alloys thereof, and also may include metals ofGroups IVB, VB, and VIB and alloys thereof. Valve metals are described,for example, by Diggle, in “Oxides and Oxide Films”, Vol. 1, pages94-95, 1972, Marcel Dekker, Inc., New York.

Tantalum and niobium are generally extracted from their ores in the formof powders. Tantalum powders, for example, that are suitable for use inhigh performance capacitors, can be produced by chemical reduction, suchas sodium reduction, of potassium fluorotantalate. In this process, thepotassium fluorotantalate is recovered from processed ore in the form ofa dry crystalline powder. The potassium fluorotantalate is melted andreduced to tantalum metal powder by sodium reduction. The tantalumpowder formed is then water washed and acid leached. Dried tantalumpowder is then recovered, thermally agglomerated at temperatures up toabout 1500° C., and crushed to a granular consistency. Typically, thegranular powder is then deoxidized in the presence of a getter materialhaving a higher affinity for oxygen than the valve metal at elevatedtemperatures up to about 1000° C., and is then acid leached to removeresidual metal contaminants and their oxides. The powder is then dried,compressed to form a pellet, sintered to form a porous body, andsubjected to anodization in a suitable electrolyte to form a continuousdielectric oxide film on the sintered body. In an alternative method,the powder is produced by hydriding a melted tantalum ingot, milling thehydrided chips, and dehydriding. In all cases it is possible, andsometimes desirable, to deoxidize the sintered anode pellet in a processsimilar to that described above for the powder.

Valve metal powders, particularly powders of tantalum, niobium, andtheir alloys, that are suitable for making capacitors should provide anadequate electrode surface area when compressed and sintered. The ufV/gof the capacitor is proportional to the surface area of the sinteredporous body. The greater the specific surface area after the sinteringoperation, the greater the ufV/g. The purity of the powder is also animportant consideration in its use in capacitor production. Metallic andnon-metallic contamination can degrade the dielectric oxide film on thecapacitors. While high sintering temperatures can be used to remove somevolatile contaminants, the high temperatures may also shrink the porousbody and its net specific surface area and, therefore, the capacitanceof the resulting capacitor. Therefore, it is important to minimize theloss of specific surface area under sintering conditions.

In the production of tantalum capacitors, for example, tantalum powderis typically heated under vacuum to cause agglomeration of the powderwhile avoiding oxidation of the tantalum. Following this treatment,however, the tantalum powder often picks up a considerable amount ofadditional oxygen because the initial surface layer of oxide goes intosolution in the metal during the heating and a new surface layer formsupon subsequent exposure to air, thereby adding to the total oxygencontent of the powder. During the later processing of these powders intoanodes for capacitors, the dissolved oxygen may recrystallize as asurface oxide and contribute to voltage breakdown or high currentleakage of the capacitor by shorting through the dielectric layer ofamorphous oxide.

As the technology of capacitors is continually demanding higher surfacearea valve metal powders, the requirement for oxygen management exceedsthe effectiveness of the available methods of oxygen control.Accordingly, the electrical properties of capacitors could be improvedif the oxygen content could be controlled, i.e., decreased or maintainedabout constant, during the powder processing.

One method to deoxidize valve metal powders, such as tantalum powder, isto mix alkaline earth metals, aluminum, yttrium, carbon, and tantalumcarbide with the tantalum powder. However, the alkaline earth metals,aluminum, and yttrium form refractory oxides that must be removed, suchas by acid leaching, before the material can be used to producecapacitors. Typically, the post-deoxidation acid leaching is performedusing a strong mineral acid solution including, for example,hydrofluoric acid, at elevated temperatures of up to 100° C. to dissolvethe refractory oxide contaminants. The carbon content must be controlledbecause it may also be deleterious to capacitors even at levels as lowas 50 ppm. Other methods have been proposed, including using athiocyanate treatment, or a reducing atmosphere throughout the tantalumpowder processing, to prevent oxidation and provide low oxygen content.

Other processes for controlling the oxygen content of valve metalmaterials, such as tantalum, niobium, and their alloys, include the useof getter materials. For example, Hard, in U.S. Pat. No. 4,722,756,describes heating the materials in an atmosphere containing hydrogen gasin the presence of a metal, such as zirconium or titanium, that is moreoxygen active than tantalum or niobium. Another process for controllingthe oxygen content of valve metal materials is disclosed by Fife, inU.S. Pat. No. 4,964,906. This process involves heating a tantalummaterial in a hydrogen-containing atmosphere in the presence of atantalum getter metal having an oxygen concentration lower than thetantalum material. While these processes provide some control of theoxygen content of valve metal materials, there is a desire to improvethe electrical properties of valve metal capacitors, particularly thoseformed from tantalum, niobium, and alloys thereof, by controlling, i.e.,decreasing or maintaining about constant, the oxygen content of thevalve metal powders. Accordingly, a demand exists for processimprovements to reduce the oxygen content of these materials,particularly after they have been subjected to a deoxidation process.

In addition to the problems with powders and capacitor applications,high oxygen contents in fabricated wrought products of valve metals candecrease the ductility of the products.

It is therefore an object of the present invention to provide a methodof controlling the oxygen content in valve metal materials. It isanother object of the present invention to provide a method ofcontrolling the oxygen content in valve metal powders, such as tantalum,niobium, and alloys thereof, useful in the production of capacitors,particularly after the powders have been subjected to a deoxidationprocess.

SUMMARY OF THE INVENTION

Accordingly, the present invention is directed to a method ofcontrolling the oxygen content in valve metal materials, such as powdersof tantalum, niobium, and their alloys. The method includes leaching adeoxidized valve metal material in an acid leach solution at atemperature lower than room temperature. In one embodiment, the methodfor controlling the oxygen content in valve metal materials includesdeoxidizing a valve metal material, preparing and cooling an acid leachsolution to a temperature lower than room temperature, and leaching thedeoxidized valve metal material in the cooled acid leach solution. Themethod of the present invention has been found to lower both the oxygenand fluoride concentrations in valve metal materials, as the use ofreduced acid leach temperatures provide lower oxygen for a givenquantity of a leach acid, such as hydrofluoric acid.

Another aspect of the present invention is directed to a method ofproducing a valve metal material, such as tantalum, niobium, and alloysthereof, having a controlled oxygen content. The method includes forminga valve metal powder, and agglomerating the powder. The agglomeratedvalve metal powder is then deoxidized in the presence of a gettermaterial that has a higher affinity for oxygen than the valve metal. Thedeoxidized valve metal is then leached in an acid leach solution at atemperature lower than room temperature to remove any getter materialcontaminants. In a further aspect of the invention, the leached valvemetal powder is washed and dried. The powder is then compressed to forma pellet, that is sintered to form a porous body. The body is thenanodized in an electrolyte to form a dielectric oxide film on the pelletsurface. In another aspect of the present invention, a sintered body isreacted with a getter (reactive) material, such as magnesium, that has ahigher affinity for oxygen than the valve metal. The sintered body isthen leached in an acid leach solution at a temperature lower than roomtemperature, and anodized in an electrolyte to form an oxide film.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is directed to a method for controlling, i.e.,decreasing or maintaining about constant, the oxygen content in valvemetal materials, such as tantalum, niobium, and their alloys, that areuseful in the production of capacitors, sintered anode bodies, andwrought products made from tantalum, niobium, and alloys thereof. Themethod includes leaching a deoxidized valve metal material in an acidleach solution at a temperature lower than room temperature.

As noted above, capacitor grade valve metal powders can be produced byseveral methods, including the chemical reduction of their ores, or byelectron beam or vacuum arc melting of a valve metal ingot. In thechemical reduction of a valve metal powder, such as tantalum powder,potassium fluorotantalate is recovered, melted and reduced to tantalummetal powder by sodium reduction. Dried tantalum powder is thenrecovered, thermally agglomerated under vacuum to avoid oxidation of thetantalum, and crushed. Because the oxygen concentration of the valvemetal material is critical in the production of capacitors, the granularpowder is then deoxidized at temperatures up to about 1000° C. in thepresence of a getter material, such as magnesium, that has a higheraffinity for oxygen than the valve metal. The powder is then acidleached to remove contaminants, including magnesium and magnesium oxide,before the material is used to produce capacitors. Typically, the acidleaching is performed using a strong mineral acid solution including,for example, hydrofluoric acid, nitric acid, sulfuric acid, hydrochloricacid, and the like, at elevated temperatures of up to 100° C. todissolve any metal and metal oxide contaminants. Preferably, nitric acidand/or hydrofluoric acid are used in the leach solution due to theirability to dissolve most metal and metal oxide contaminants, as well asvalve metal fines. The powder is then washed and dried, compressed toform a pellet, sintered to form a porous body, and anodized in asuitable electrolyte to form a continuous dielectric oxide film on thesintered body. In some cases the sintered body is deoxidized withmagnesium in a process similar to the powder treatment prior to beinganodized.

The deoxidation process is typically followed by a mineral acid leachingprocess to dissolve any contaminants. In addition, it is furtherrecognized that the leach solution, including hydrofluoric acid, mayfurther lower the oxygen concentration by dissolving very small valvemetal particles (fines). However, use of hydrofluoric acid may providean undesirable increase in the fluoride concentration on the resultingparticle and, accordingly, undesirable corrosion on process equipment.Typically, the mineral acid solution contains less than 10%, by weight,hydrofluoric acid. Preferably, less than 5%, by weight, of hydrofluoricacid is used in the acid leach solution to dissolve residual metal andmetal oxide contaminants while minimizing fluoride concentration; mostpreferably, less than 1%, by weight, of hydrofluoric acid is used. It isnoted, however, that a leach solution containing no hydrofluoric acid,so as to eliminate fluoride contamination, is also desirable, providedthe solution is effective at lowering the oxygen concentration of thevalve metal particles by dissolving contaminants and fines.

Elevated temperatures (above room temperature to about 100° C.) aretraditionally used during the post-deoxidation acid leach to increasethe activity of the acid solution in dissolving any residual metal andmetal oxide contaminants, such as magnesium and magnesium oxide, on thevalve metal material. The high temperature post-deoxidation acid leachalso etches the valve metal particles and increases their surface area,thereby resulting in an undesirable increase in oxygen concentrationupon subsequent exposure to the atmosphere. As a result, furtherprocessing may be necessary to control the oxygen concentration of thevalve metal materials to ensure their suitability for capacitor andrelated applications.

The process of the present invention, however, performs thepost-deoxidation acid leaching at temperatures lower than roomtemperature to minimize the leach effect on the particle surface area,i.e. remove the residual metal and metal oxide contaminants whilecontrolling deleterious etching, and the increase in the oxygenconcentration, of the valve metal materials. As known to those skilledin the art, “room temperature” generally means an indoor temperature ofbetween about 20° C. and about 25° C. (between about 68° F. and about77° F.). Because the chemical reactions during acid leaching areexothermic, the starting leach temperature is often the lowesttemperature in the process; it may be measured prior to the addition ofthe valve metal, after the addition of the valve metal, or during theacid leaching. Most typically, the leach temperature is the acid leachsolution temperature prior to the addition of the valve metal material.In the cases of the present examples (described below) the acid leachtemperature is defined as the temperature of the acid leaching solutionprior to the addition of the deoxidized valve metal material.

It should be understood that lowering the temperature at the start ofthe acid leach procedure results in a generally lower temperaturethroughout the process than would have been measured if the solution wasat, or greater than, room temperature prior to the addition of the valvemetal material. For large scale leaches, wherein a large amount of heatenergy will be liberated, active cooling must be employed to extract theheat. In a small scale acid leach, the reactants (leach solution and/orvalve metal material) can be cooled prior to being mixed to effectivelyextract the heat. The acid leach solution is prepared and cooled usingtechniques known to those skilled in the art. For example, the acidsolution, and/or the valve metal material, may be precooled, the acidleaching container may be precooled, and/or ice may be added to the acidleach solution after the solution has been added to a leach container.It has been found that acid leach solutions at temperaturessubstantially below room temperature are most effective to removeresidual metal and metal oxide contaminants while controlling theresulting oxygen concentration of the valve metal materials. A preferredacid leach solution temperature is below about 25° C.; most preferably,the acid leach solution temperature is below about 0° C. to effectivelyremove the heat of reaction between the acid leach solution and theresidual metal and metal oxide contaminants, and slow the leach solutioneffect on the surface of the valve metal material.

Although the process of the present invention is effective to controlundesirable oxygen concentrations, it is noted that a minimumconcentration of oxygen will remain on the valve metal particles duringnormal processing due to their high affinity for oxygen. This level willtypically be sufficient to passivate the surface of the particle. In theproduction of capacitor grade valve metal powders, lower levels ofoxygen on the valve metal particles are preferred. For example, tantalumpowders for use in capacitors preferably have less than 3000 ppm, andmore preferably less than 2400 ppm, oxygen. Similar levels of oxygen onsintered tantalum electrode bodies have been found to be acceptable.

The present invention will be further illustrated by the followingexamples which are intended to be illustrative in nature and are not tobe considered as limiting the scope of the invention.

EXAMPLE I

Variations in the concentration of the hydrofluoric acid (HF), theconcentration of the nitric acid (HNO₃), and the temperature of apost-deoxidation acid leach of a tantalum powder was evaluated.

The HF concentration (ml/lb of tantalum powder leached), leachtemperature (° C.), and HNO₃ concentration (wt %) were varied todetermine the optimum leaching conditions. These factors were variedusing C255 grade tantalum powder, available from the Cabot PerformanceMaterials Division of Cabot Corporation, Boyertown, Pa. The C255 gradetantalum powder is a mid- to high- voltage flaked powder for use at15,000 to 18,000 CV/g.

The tantalum powder was prepared by first cooling a 600 milliliterplastic leach container by placing it in stainless steel tray containinga bath of ice cubes and coarse salt. About 250 milliliters of deionizedwater was added to the leach container. About 125 milliliters of areagent grade HNO₃, having a concentration of between about 68% andabout 70%, was then added into the leach container slowly underagitation. A 2 inch diameter plastic coated propeller-type agitator, setat about 425 rpm, was used to mix the liquids. The temperature of theHNO₃/water was cooled to, and maintained at, about 20° C. After thedesired temperature was achieved, about 1 pound of C255 grade flakedtantalum powder was added into the leach container during agitation.Prior to its addition to the leach container, the tantalum powder wassubjected to a magnesium deoxidation process. After the tantalumaddition, about 5 milliliters of a reagent grade HF, having aconcentration of between about 48% and about 51%, was then added intothe leach container slowly under agitation. After the HF addition, theleach container contents were mixed for about 30 minutes.

After the tantalum powder was leached for about 30 minutes, the agitatorwas turned off and the temperature was measured to be about 5° C. Thetantalum powder was then allowed to settle, and the acid was decanted.The tantalum powder was then transferred into a 4,000 milliliter plasticcontainer and washed using room temperature deionized water. Thetantalum powder was then allowed to settle, and the wash water wasdecanted. The washing step was repeated until the conductivity of thedecanted wash water was less than 10 μMohs/cm. The water conductivitywas measured using a Cole-Parmer Model 1500-00 conductivity meter.

After the desired water conductivity was reached, the tantalum solutionwas filtered using a Buchner funnel, filter paper, and a vacuum pump.The damp tantalum powder was recovered and transferred into a stainlesssteel pan. The powder was then dried in a vacuum oven at about 180° F.(about 82° C.) for about 6 hours. The dried tantalum powder was thenscreened through a 50 mesh sieve and analyzed. The foregoing process wasrepeated using portions from the same blended lot of deoxidized tantalumpowder, varying the HF concentration, leach temperature (defined as theHNO₃/water solution temperature prior to tantalum addition), and HNO₃concentration, to determine the optimum leaching conditions to controloxygen content in tantalum powder. The ranges of each variable(including HF, HNO₃, and leach temperature) and the experimental results(fluoride and oxygen concentration, and BET surface area measured usingASTM method D4567 continuous N₂ flow) are listed below in Table 1.

TABLE 1 Sample HF HNO₃ Temp. F Oxygen BET No. (ml/lb Ta) (wt %) (° C.)(ppm) (ppm) (m²/g) 1 1 23.000 20 24 1888 0.52 2 1 70.000 20 32 2061 0.553 5 23.000 20 69 1433 0.44 4 5 70.000 20 131 1498 0.51 5 1 23.000 80 372490 0.79 6 1 70.000 80 35 2491 0.86 7 5 23.000 80 144 2725 1.19 8 570.000 80 301 4183 2.00

As reported in Table 1, a reduced temperature acid leach results incontrolled oxygen content in the final tantalum powder. Samples 1through 4 were evaluated at an acid leach temperature of 20° C., whilevarying the HF content between 1 and 5 milliliter HF per pound oftantalum (Samples 1 and 2, and Samples 3 and 4, respectively), andadjusting the HNO₃ concentration between 23.0 and 70.0 percent, byweight, between samples. As expected, of the Samples 1 through 4, alower oxygen content was measured with the Sample 3 and 4 tantalummaterials due to the additional HF content, which dissolved the smallesttantalum particles (fines) from the tantalum material. It is noted thateach of the Samples 1 through 4, utilizing a reduced temperature leach,controlled the oxygen content to acceptable levels (less than about 2100ppm oxygen). The materials produced with less HF addition are preferred.The adjustment of the HNO₃ concentration (between Samples 1 and 2, and 3and 4) appears to have only a minimal effect upon the oxygen content ofthe final tantalum powder.

Samples 5 through 8 were evaluated at an acid leach temperature of 80°C., while varying the HF content between 1 and 5 milliliter HF per poundof tantalum (Samples 5 and 6, and Samples 7 and 8, respectively), andadjusting the HNO₃ concentration between 23.0 and 70.0 percent, byweight, between samples. Each of these Samples exceeded an oxygen rangeof about 2400 ppm. However, a lower oxygen content was measured with theSample 5 and 6 tantalum materials that utilized a lower HF contentbecause, at high temperature, the increase in surface area due toetching by HF predominates over the removal of very small particles.

The overall results indicate that the fluoride level of the final powderis determined by the amount of HF used in the acid leach. Moreover, asexpected, the surface areas of the particles are proportional to theoxygen content of the final powder.

Therefore, it is seen that the use of low leach temperatures isimportant to lower both oxygen and fluoride together, because lowtemperature provides lower oxygen for a given HF amount, and the lowestpossible HF amount is needed to control the fluoride content in thefinal powder.

EXAMPLE II

Variations in the concentration of the hydrofluoric acid (HF) (ml/lb oftantalum powder leached), and the temperature of a post-deoxidation acidleach of a tantalum powder was evaluated to determine the optimumleaching conditions to control the oxygen content of the powder.

These factors were varied using C515 grade tantalum powder, availablefrom the Cabot Performance Materials Division of Cabot Corporation,Boyertown, Pa. The C515 grade tantalum powder is a low- to mid-voltagenodular-shaped powder for use at 35,000 to 45,000 CV/g.

The tantalum powder was prepared by premixing 1 liter of reagent gradeHNO₃, having a concentration of between about 68% and about 70%, andabout 2 liters of deionized water in a container. The temperature of theHNO₃/water solution was cooled by placing the container in a foam chestcontaining a bath of ice cubes and coarse salt. A plastic coated steelbarrel with baffles, used as a leach container, having a volume of about100 liters, was then prechilled by adding between about 8 and 10 poundsof ice and enough deionized water to cover the ice in the leachcontainer. The container was then rotated for about 10 minutes, theice/water was poured off, and the container was rinsed with deionizedwater. The HNO₃/water solution temperature was then measured with athermocouple to be about 0° F. (about −16° C.). The HNO₃/water solutionwas then added to the prechilled leach container, and about 5 pounds ofC515 grade nodular tantalum powder was added into the leach containerduring agitation. Prior to its addition to the leach container, thetantalum powder was subjected to a magnesium deoxidation process, andwas screened to about −50 mesh to remove any lumps. After the tantalumaddition, a reagent grade HF, having a concentration of between about49% and about 51%, was then added into the leach container slowly underagitation. After the HF addition, the leach container contents weremixed for about 30 minutes.

After the tantalum powder was leached for about 30 minutes, the agitatorwas turned off. The tantalum powder was then allowed to settle for about10 minutes after additional deionized water was added, and theacid/water was decanted. The tantalum powder was then washed using roomtemperature deionized water, and a 2 minute rotation period. Thetantalum powder was then allowed to settle, and the wash water wasdecanted. The washing step was repeated until the conductivity of thedecanted wash water was less than 10 μMohs/cm. The water conductivitywas measured using a Cole-Palmer Model 1500-00 conductivity meter.

After the desired water conductivity was reached, the water was decantedand the tantalum powder was filtered. The damp tantalum powder wasrecovered and transferred into a stainless steel pan. The powder wasthen dried in a vacuum oven set at about 180° F. (about 82° C.) forabout 6 hours. The foregoing process was repeated, varying the HFconcentration, and the leach temperature (defined as the HNO₃/watersolution temperature prior to tantalum addition) to determine theoptimum leaching conditions to control oxygen content in tantalumpowder. The ranges of each variable (including HF and leachtemperature), and the experimental results (fluoride and oxygenconcentration, and BET surface area measured using ASTM method D4567continuous N₂ flow) are listed below in Table 2.

TABLE 2 Sample Temperature F BET Oxygen No. HF (ml.lb Ta), (° C.) (ppm)(m²/g) (ppm) 1 1 −12.0 <60.0 0.63 2289 2 5 −16.0 88.0 0.71 2021 3 1 31.0<68.0 0.81 2742 4 5 33.0 112.0 0.69 1884

As reported in Table 2, a reduced temperature acid leach results incontrolled oxygen content in the final tantalum powder. Samples 1 and 2were evaluated using 1 and 5 milliliter HF per pound of tantalum,respectively, at acid leach temperatures of −12.0° C. and −16.0° C. Asexpected, a lower oxygen content was measured with the Sample 2 tantalummaterial due to the additional HF content, which dissolved additionalsmall tantalum particles. As a result of the additional HF, however, thefluoride content of the Sample 2 tantalum material is higher. Becausethe oxygen content of Samples 1 and 2 is controlled, the materialproduced with reduced BF content (Sample 1) is preferred due to thelower resulting fluoride content.

Although the lowest oxygen content was measured with the Sample 4tantalum material, it is a result of the high level of HF in the leachsolution and, accordingly, the reduced surface area. An undesirably highlevel of fluoride was also measured in Sample 4. As noted above,elevated temperatures are known to increase the activity of the acidsolution in dissolving contaminants on the valve metal material. Thecombination of increased HF content at an elevated temperature in Sample4, however, resulted in reduced surface area. A lower quantity of HF inthe acid leach solution at an elevated temperature in Sample 3 resultedin an increase in surface area because the particle surface was etchedand not dissolved. This increase in surface area resulted in an oxygencontent of more than 2700 ppm.

The foregoing results also confirm that the fluoride level is determinedby the amount of HF used in the acid leach. The same quantity of HF (1ml/lb Ta) was used in Samples 1 and 3, and Samples 2 and 4, whilevarying the leach temperature. As reported, while reduced temperatureslower the oxygen content to acceptable levels, the fluoride content wasonly marginally reduced. It is noted, however, that varying the HFcontent, as between Samples 1 and 2, and Samples 3 and 4 (1 and 5 ml/lbTa, respectively) and using reduced temperatures for Samples 1 and 2,and elevated temperatures for Samples 3 and 4, resulted in higherfluoride levels in Samples 2 and 4, which used higher levels of HF inthe acid leach solution.

Therefore, it is seen that the use of reduced acid leach solutiontemperatures is important to lower both oxygen and fluoride together,because low temperatures provide lower oxygen for a given HF amount, andthe lowest possible HF amount is needed to control the fluoride contentin the final powder.

EXAMPLE III

Variations in the temperature of a post-deoxidation acid leach of aniobium powder was evaluated to determine the optimum leachingconditions to control the oxygen content of the powder.

The temperature of the acid leach was varied using a deoxidized WCb-Cgrade niobium powder, available from the Cabot Performance MaterialsDivision of Cabot Corporation, Boyertown, Pa. The WCBC grade niobiumpowder is an ingot-derived, low surface area powder. The WCb-C gradeniobium powder was first deoxidized by blending a 1 kilogram sample with0.4% magnesium in a tantalum tray. The tray was then covered, placedinto a retort, and heated in a furnace at a temperature of 750° C. in anargon atmosphere, for about 1 hour. After this period, a vacuum wasapplied to the retort, the argon was removed, and a final pressure ofless than about 400 microns was achieved and held for about 1 hour. Theretort was then cooled to a temperature of less than about 200° C., andwas removed from the furnace. After the system cooled to a temperatureof less than about 40° C., it was passivated by adding air beforeopening the retort and removing the niobium powder. The resultingdeoxidized niobium powder had an oxygen content of 1767 ppm.

The deoxidized niobium powder was then treated in three differenttemperature acid leaches to determine the effectiveness of the acidleach temperature on controlling the oxygen content of the powder. Theacid leach solution was prepared by premixing about 55 milliliters ofreagent grade HNO₃, having a concentration of between about 68%, andabout 110 milliliters of deionized water (resulting in a 165 ml solutionof 23% HNO₃) in a 250 milliliter plastic container. About 100 grams ofthe deoxidized WCb-C grade niobium powder was then added to the leachcontainer during agitation. After the niobium powder addition, about 0.9milliliter of a reagent grade HF, having a concentration of about 49%,was then added into the leach container slowly under agitation. Afterthe HF addition, the leach container contents were mixed for about 30minutes.

After the niobium powder was leached for about 30 minutes, the agitatorwas turned off. The niobium powder was then allowed to settle for about10 minutes after additional deionized water was added, and theacid/water was decanted. The niobium powder was then washed using roomtemperature deionized water. The niobium powder was then allowed tosettle, and the wash water was decanted. The washing step was repeateduntil the conductivity of the decanted wash water was less than 10μMohs/cm.

After the desired water conductivity was reached, the water was decantedand the niobium powder was filtered. The damp niobium powder was thenrecovered and dried in a vacuum oven at about 85° C. The foregoingprocess was repeated, varying the leach temperature (defined as theHNO₃/water solution temperature prior to niobium powder addition) todetermine the optimum leaching temperature to control oxygen content inniobium powder. The niobium powder was added to the 23% HNO₃ solution attemperatures of about 30° C., about 3° C., and about 55° C. The 3° C.acid leach solution was prepared by cooling the 23% HNO₃ solution in anice/salt bath; the 55° C. acid leach solution was prepared by usingheated deionized water (about 60° C.) to form the acid/water leachsolution, and using a hot water bath (between about 45° C. and about 50°C.) to maintain the elevated temperature. The experimental results(oxygen concentration) are listed below in Table 3.

TABLE 3 Oxygen (ppm) Oxygen (ppm) Oxygen (ppm) Sample No. (30° C.) (3°C.) (55° C.) 1 379 234 773 2 595 558 1007 3 64g 574 968 4 558 431 791 5672 567 962 Average 570 473 900

As reported in Table 3, a cooled acid leach solution results in reducedoxygen content in the final niobium powder. The powder that was leachedin the 3° C. acid leach solution had an average oxygen content of 473ppm, which was about 100 ppm less than the powder that was leached inthe 30° C. acid leach solution. The powder that was leached in thewarmest acid solution (about 55° C.) had an average oxygen content of900 ppm, which is 330 ppm more than the powder that was leached atnear-ambient temperature, and almost double the oxygen content of thepowder that was leached in the coldest acid leach solution. Therefore,the use of reduced acid leach temperatures is important to control(lower) the oxygen content in deoxidized valve metal materials such asniobium powder.

EXAMPLE IV

Variations in the temperature of an acid leach of a non-deoxidizedtantalum powder was evaluated to determine the optimum leachingconditions to control the oxygen content of the powder.

The temperature of the acid leach was varied using a non-deoxidized C275grade tantalum powder, available from the Cabot Performance MaterialsDivision of Cabot Corporation, Boyertown, Pa. The non-deoxidizedtantalum powder had an oxygen content of8913ppm.

The acid leach solution was prepared by premixing about 33 millilitersof reagent grade HNO₃, having a concentration of between about 68%, andabout 66 milliliters of deionized water (resulting in a 99 ml solutionof 23% HNO₃) in a 250 milliliter plastic container. A cold leachsolution (about −3° C.) was prepared by cooling the 23% HNO₃ solution inan ice/salt bath. About 120 grains of the non-deoxidized C275 gradetantalum powder was added to the leach container during agitation. Afterthe tantalum powder addition, about 0.3 milliliter of a reagent gradeHF, having a concentration of about 49%, was then added into the leachcontainer slowly under agitation. After the HF addition, the leachcontainer contents were mixed for about 30 minutes. A second leachsolution (about 37° C.), prepared by using warm deionized water, wasalso evaluated to treat about 120 grams of the non-deoxidized tantalumpowder as described above.

After the tantalum powder was leached for about 30 minutes, the agitatorwas turned off. The tantalum powder was then allowed to settle for about10 minutes after additional deionized water was added, and theacid/water was decanted. The tantalum powder was then washed using roomtemperature deionized water. The tantalum powder was then allowed tosettle, and the wash water was decanted. The washing step was repeateduntil the conductivity of the decanted wash water was less than10μMohs/cm.

After the desired water conductivity was reached, the water was decantedand the tantalum powder was filtered. The damp tantalum powder was thenrecovered and dried in a vacuum oven at about 85° C. The foregoingprocess was repeated for each leach solution to determine the optimumleaching temperature to control oxygen content in a non-deoxidizedtantalum powder. The experimental results (oxygen concentration) arelisted below in Table 4.

TABLE 4 Oxygen (ppm) Oxygen (ppm) Sample No. (−3° C.) (37° C.) 1 90378477 2 9112 8818 3 9198 8994 4 7599 8824 5 8794 8870 Average 8748 8797

As reported in Table 4, neither the cold nor the warm acid leachsignificantly lowered the oxygen content of the non-deoxidized tantalumpowder. The powder that was leached in the reduced temperature acidleach solution had an average oxygen content of 8748 ppm, and the powderthat was leached in the wanner solution had an average oxygen content of8797 ppm. As noted above, the oxygen content of the startingnon-deoxidized tantalum powder was 8913 ppm. The use of reduced acidleach temperatures, therefore, appears to be ineffective in controlling(lowering) the oxygen content in non-deoxidized valve metal materialssuch as tantalum powder.

EXAMPLE V

Variations in the temperature of an acid leach of a sintered tantalumanode was evaluated to determine the optimum leaching conditions tocontrol the oxygen content of the anode.

The temperature of the acid leach was varied using a sintered anodesmade from HP110 finished tantalum powder, available from the CabotPerformance Materials Division of Cabot Corporation, Boyertown, Pa. Theanodes weighed 476 grams each, with a press density of 5.0 g/cc, andwere sintered at 1570° C. for 30 minutes. The anodes were cut into smallpieces before leaching.

The acid leach solution was prepared by premixing about 10 millilitersof reagent grade HNO₃, having a concentration of between about 68%, andabout 20 milliliters of deionized water (resulting in a 30 ml solutionof 23% HNO₃) in a 100 milliliter plastic container. A cold leachsolution (about −3° C.) was prepared by cooling the 23% HNO₃ solution inan ice/salt bath. About 3.5 grams of the tantalum anode pieces wereadded to the leach container during agitation. After the tantalum anodeaddition, about 0.05 milliliter of a reagent grade HF, having aconcentration of about 49%, was then added into the leach containerslowly under agitation. After the HF addition, the leach containercontents were mixed for about 30 minutes. A second leach solution (about42° C.), prepared by using warm deionized water, was also evaluated totreat about 3.5 grams of the tantalum anode pieces as described above.

After the tantalum anode pieces were leached for about 30 minutes, theagitator was turned off. The tantalum anode pieces were then allowed tosettle for about 10 minutes after additional deionized water was added,and the acid/water was decanted. The tantalum anode pieces were thenwashed using room temperature deionized water. The tantalum anode pieceswere then allowed to settle, and the wash water was decanted. Thewashing step was repeated until the conductivity of the decanted washwater was less than 10 μMohs/cm.

After the desired water conductivity was reached, the water was decantedand the tantalum anode pieces were recovered and dried in a vacuum ovenat about 85° C. The foregoing process was repeated for each leachsolution to determine the optimum leaching temperature to control oxygencontent in a sintered tantalum anodes. The experimental results (oxygenconcentration) are listed below in Table 5.

TABLE 5 Oxygen (ppm) Oxygen (ppm) Oxygen (ppm) Sample No. Pre-Leach (−3°C.) (42° C.) 1 2630 2435 2705 2 2432 2472 2401 3 2502 2486 2331 4 24442466 2390 5 2424 2543 2114 6 2488 2534 2309 7 2446 2619 2488 8 2651 25002438 9 2475 2651 2465 10 2552 2537 2491 11 2557 2604 2531 12 2605 18842617 Average 2517 2476 2441

As reported in Table 5, neither the cold nor the warm acid leachsignificantly lowered the oxygen content of the sintered tantalum anodepieces. The sintered tantalum anode pieces that were leached in the thereduced temperature acid leach solution had an average oxygen content of2476 ppm, and the powder that was leached in the warmer acid leachsolution had an average oxygen content of 2441 ppm. The average oxygencontent of the sintered tantalum anode pieces was 2517 ppm. The use ofreduced acid leach solution temperatures, therefore, appears to beineffective in controlling (lowering) the oxygen content of sinteredvalve metal materials, such as tantalum anodes, relative to warmer acidleach solutions.

EXAMPLE VI

Variations in the temperature of an acid leach of an ingot-derivedniobium powder was evaluated to determine the optimum leachingconditions to control the oxygen content of the powder.

The temperature of the acid leach was varied using a non-deoxidized,ingot-derived WCb-C niobium powder, available from the Cabot PerformanceMaterials Division of Cabot Corporation, Boyertown, Pa. The powder wasproduced by hydriding and crushing a niobium ingot. The powder was thendegassed in a vacuum oven.

The acid leach solution was prepared by first premixing about 55milliliters of reagent grade HNO₃, having a concentration of betweenabout 68%, and about 110 milliliters of deionized water (resulting in a165 ml solution of 23% HNO₃) in a 250 milliliter plastic container. Acold leach solution (about 0° C.) was prepared by cooling the 23% HNO₃solution in an ice/salt bath. About 200 grams of the niobium powder wasadded to the leach container during agitation. After the niobium powderaddition, about 0.5 milliliter of a reagent grade HF, having aconcentration of about 49%, was then added into the leach containerslowly under agitation. After the HF addition, the leach containercontents were mixed for about 30 minutes. A second leach solution (about38° C.), prepared by using warmed deionized water, was also evaluated totreat about 200 grams of the niobium powder as described above.

After the niobium powder was leached for about 30 minutes, the agitatorwas turned off. The niobium powder was then allowed to settle for about10 minutes after additional deionized water was added, and theacid/water was decanted. The niobium powder was then washed using roomtemperature deionized water. The niobium powder was then allowed tosettle, and the wash water was decanted. The washing step was repeateduntil the conductivity of the decanted wash water was less than 10μMohs/cm.

After the desired water conductivity was reached, the water was decantedand the niobium powder was filtered. The damp niobium powder wasrecovered and was dried in a vacuum oven at about 85° C. The foregoingprocess was repeated for each leach solution to determine the optimumleaching temperature to control oxygen content in an ingot derivedniobium powder. The experimental results (oxygen concentration) arelisted below in Table 6.

TABLE 6 Oxygen (ppm) Oxygen (ppm) Oxygen (ppm) Sample No. Pre-Leach (0°C.) (38° C.) 1 2337 1883 1636 2 2481 1925 1777 3 2412 2045 1874 4 — 20601984 5 — 1582 1511 Average 2410 1899 1756

As reported in Table 6, the cold acid leach did not significantly lowerthe oxygen content of the ingot-derived niobium powder relative to thewarmer acid leach. The powder that was leached in the the reducedtemperature acid leach solution had an average oxygen content of 1899ppm, and the powder that was leached in the warmer acid leach solutionhad an average oxygen content of 1756 ppm. The average oxygen content ofthe ingot-derived niobium powder was 2410 ppm. The use of reduced acidleach solution temperatures, therefore, appears to be ineffective incontrolling (lowering) the oxygen content of non-deoxidized,ingot-derived valve metal materials, such as niobium powder, relative towarmer acid leach solutions.

Although particular embodiments of the invention have been described indetail for purposes of illustration, various changes and modificationsmay be made without departing from the scope and spirit of theinvention. For example, the process of the present invention may also beused to control the oxygen content of wrought products of valve metals.Accordingly, the invention is not to be limited except as by theappended claims.

What is claimed is:
 1. A method for controlling the oxygen content invalve metal materials, comprising: leaching a deoxidized valve metalmaterial in an acid leach solution at a temperature lower than roomtemperature.
 2. The method of claim 1, wherein said acid leach solutionis cooled to a temperature lower than room temperature prior to leachingsaid deoxidized valve metal material.
 3. The method of claim 1, whereinsaid valve metal is selected from the group consisting of tantalum,niobium, and alloys thereof.
 4. The method of claim 3, wherein saidvalve metal is tantalum.
 5. The method of claim 3, wherein said valvemetal is niobium.
 6. The method of claim 1, wherein said valve metalmaterial is selected from the group consisting of nodular powders,flaked powders, ingot-derived powders, and sintered bodies.
 7. Themethod of claim 1, wherein said acid leach solution temperature is lessthan about 25° C.
 8. The method of claim 7, wherein said acid leachsolution temperature is less than about 0° C.
 9. The method of claim 1,wherein said acid leach solution comprises a mineral acid.
 10. Themethod of claim 9, wherein said acid leach solution comprises less than10%, by weight, hydrofluoric acid.
 11. A method of producing a valvemetal material having a controlled oxygen content, comprising the stepsof: forming a valve metal powder; agglomerating said valve metal powder;deoxidizing said agglomerated valve metal powder in the presence of agetter material that has a higher affinity for oxygen than said valvemetal powder; and leaching said deoxidized valve metal powder in an acidleach solution at a temperature lower than room temperature to removegetter material contaminants.
 12. The method of claim 11, wherein saidacid leach solution is cooled to a temperature lower than roomtemperature prior to leaching said deoxidized valve metal powder. 13.The method of claim 11, wherein said valve metal is selected from thegroup consisting of tantalum, niobium, and alloys thereof.
 14. Themethod of claim 13, wherein said valve metal is tantalum.
 15. The methodof claim 13, wherein said valve metal is niobium.
 16. The method ofclaim 11, wherein said valve metal powder is thermally agglomeratedunder vacuum.
 17. The method of claim 11, wherein said valve metalpowder is deoxidized at temperatures up to about 1000°C. in the presenceof a getter material comprising magnesium.
 18. The method of claim 11,wherein said acid leach solution comprises a mineral acid.
 19. Themethod of claim 18, wherein said acid leach solution comprises less than10%, by weight, hydrofluoric acid.
 20. The method of claim 11, furthercomprising the steps of: washing and drying said acid leached valvemetal powder; compressing said powder to form a pellet; sintering saidpellet to form a porous body; and anodizing said porous body in anelectrolyte to form a continuous dielectric oxide film on said porousbody.
 21. The method of claim 20, further comprising the steps of:deoxidizing said sintered porous body in the presence of a gettermaterial that has a higher affinity for oxygen than said valve metal;and leaching said sintered porous body in an acid leach solution at atemperature lower than room temperature to remove getter materialcontaminants prior to anodizing said porous body.
 22. The method ofclaim 11, wherein said acid leach solution temperature is less thanabout 25° C.
 23. The method of claim 22, wherein said acid leachsolution temperature is less than about 0° C.